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Register a new userGenuine Phase Diagram of Homogeneously Doped CuO2 Plane in High-Tc Cuprate Superconductors
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We report a genuine phase diagram for a disorder-free CuO_2 plane based on the precise evaluation of the local hole density (N_h) by site-selective Cu-NMR studies on five-layered high-Tc cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N_h=0.17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N_h< 0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM(Paramagnetism) phases may be present at N_h=0.15 and T=75 K, (4) Tc is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high-Tc SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the Tc of cuprate superconductors is so high.
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We propose that Resistivity Curvature Mapping (RCM) based on the in-plane resistivity data is a useful way to objectively draw an electronic phase diagrams of high-T_c cuprates, where various crossovers are important. In particular, the pseudogap crossover line can be conveniently determined by RCM. We show experimental phase diagrams obtained by RCM for Bi_{2}Sr_{2-z}La_{z}CuO_{6+delta}, La_{2-x}Sr_{x}CuO_{4}, and YBa_{2}Cu_{3}O_{y}, and demonstrate the universal nature of the pseudogap crossover. Intriguingly, the electronic crossover near optimum doping depicted by RCM appears to occur rather abruptly, suggesting that the quantum critical regime, if exists, must be very narrow.
Taking the spin-fermion model as the starting point for describing the cuprate superconductors, we obtain an effective nonlinear sigma-field hamiltonian, which takes into account the effect of doping in the system. We obtain an expression for the spin-wave velocity as a function of the chemical potential. For appropriate values of the parameters we determine the antiferromagnetic phase diagram for the YBa$_2$Cu$_3$O$_{6+x}$ compound as a function of the dopant concentration in good agreement with the experimental data. Furthermore, our approach provides a unified description for the phase diagrams of the hole-doped and the electron doped compounds, which is consistent with the remarkable similarity between the phase diagrams of these compounds, since we have obtained the suppression of the antiferromagnetic phase as the modulus of the chemical potential increases. The aforementioned result then follows by considering positive values of the chemical potential related to the addition of holes to the system, while negative values correspond to the addition of electrons.
Starting from a spin-fermion model for the cuprate superconductors, we obtain an effective interaction for the charge carriers by integrating out the spin degrees of freedom. Our model predicts a quantum critical point for the superconducting interaction coupling, which sets up a threshold for the onset of superconductivity in the system. We show that the physical value of this coupling is below this threshold, thus explaining why there is no superconducting phase for the undoped system. Then, by including doping, we find a dome-shaped dependence of the critical temperature as charge carriers are added to the system, in agreement with the experimental phase diagram. The superconducting critical temperature is calculated without adjusting any free parameter and yields, at optimal doping $ T_c sim $ 45 K, which is comparable to the experimental data.
In order to investigate the low-energy antiferromagnetic Cu-spin correlation and its relation to the superconductivity, we have performed muon spin relaxation (muSR) measurements using single crystals of the electron-doped high-Tc cuprate Pr_1-x_LaCe_x_CuO_4_ in the overdoped regime. The muSR spectra have revealed that the Cu-spin correlation is developed in the overdoped samples where the superconductivity appears. The development of the Cu-spin correlation weakens with increasing x and is negligibly small in the heavily overdoped sample where the superconductivity almost disappears. Considering that the Cu-spin correlation also exist in the superconducting electron-doped cuprates in the undoped and underdoped regimes [T. Adachi et al., J. Phys. Soc. Jpn. 85, 114716 (2016)], our findings suggest that the mechanism of the superconductivity is related to the low-energy Cu-spin correlation in the entire doping regime of the electron-doped cuprates.
Here, we report an overview of the phase diagram of single layered and double layered Fe arsenide superconductors at high magnetic fields. Our systematic magnetotransport measurements of polycrystalline SmFeAsO$_{1-x}$F$_x$ at different doping levels confirm the upward curvature of the upper critical magnetic field $H_{c2}(T)$ as a function of temperature $T$ defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single and double layered compounds. In all compounds explored by us the zero temperature upper critical field $H_{c2}(0)$, estimated either through the Ginzburg-Landau or the Werthamer-Helfand-Hohenberg single gap theories, strongly surpasses the weak coupling Pauli paramagnetic limiting field. This clearly indicates the strong coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses $gamma = (m_c/m_{ab})^{1/2}$ for carriers moving along the c-axis and the ab planes, respectively, is relatively modest as compared to the high-$T_c$ cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field $H_m(T)$, separating the vortex-solid from the vortex-liquid phase in the single layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.
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